An alternator that generates electricity in an automobile has heretofore used a diode as a rectifier. The diode is inexpensive but has a forward voltage drop causing a large power loss. Contrastingly, in recent years, MOSFET has begun to be used as the rectifier for the alternator. Synchronous rectification of the MOSFET enables developing of a rectifier that has no forward voltage drop and raises a forward current at 0 Volt and generates a small power loss.
A power supply outputs an AC power with a constant frequency, and therefore, when a MOSFET is used for a rectifier of the power supply, on-off control of the MOSFET can be performed by way of synchronization with a clock. However, the alternator outputs an AC power with a non-constant frequency with a coil, and therefore, when a MOSFET is used for a rectifier of the alternator, on-off control of the MOSFET does not require the simple synchronization with the clock like the MOSFET used for the power supply or the like, but requires synchronizations with various frequencies.
Accordingly, a method is thought out of controlling the MOSFET by way of a detection of a position of the motor by using a Hall element. However, the method using the Hall element is not able to replace a currently used rectifier without any other changes, but needs a drastic change of the alternator.
The claim 1 of Patent Literature 1 describes “A rectification circuit, comprising: a cathode terminal (K1); an anode terminal (A1); and an electronic circuit provided between the cathode terminal and the anode terminal, the electronic circuit including an MOS transistor (T1) having an inverse diode (D6) integrated, a capacitor (C1) and a differential amplifier (T2, T3, R1, R2, R3).” The paragraph 0018 of Patent Literature 1, “If the electrical potential at cathode terminal K1 of the rectification circuit is more positive than the electrical potential at anode terminal A1 of the rectification circuit and if this potential difference exceeds a value set by Zener diode D4, the input potential of the power amplifying stage consisting of transistors T4 and T5 is raised. This also increases the gate-to-source voltage at MOS transistor T1 and a current flow comes about between the drain and the source of MOS transistor T1.” Here, the MOSFET of the structure described in Patent Literature 1 is referred to as an autonomous type of MOSFET.
An autonomous type synchronous rectification MOSFET needs no sensor such as a Hall element and generally needs a simple control circuit, and thus allows a rectification part of the alternator to be configured at inexpensive cost.
Paragraph 0013 of Patent Literature 2 describes “At the first half stage of the transition period to the turn-on, the rising rate of the gate voltage at the voltage-driven element is relatively increased; contrastingly at the second half stage of the transition period to the turn-on, the rising rate of the gate voltage at the voltage-driven element is relatively decreased. This improves a trade-off in switching characteristics when the voltage-driven element is turned on.” Paragraph 0029 describes that the effect of the trade-off is ““suppressing the surge and the ringing phenomenon of the drain current when the transistor Tr1 is turned on”.”
The ringing phenomenon that is the problem to be solved by Patent Literature 2 means an oscillation generated when switching a switching element between on and off. The ringing is a phenomenon caused by inductors and parasitic capacitances in a substrate upon a high speed switching.